1. Field of the Invention
The present invention relates to lenses and optical pickups and, more particularly, to a lens which corrects a wavefront error caused by the tilt of the lens itself, an optical pickup using the lens as an objective lens, and a method of correcting a wavefront error.
2. Description of the Related Art
Optical recording and/or reproducing apparatuses can record data on or reproduce the data from an optical disc, which is an optical information storage medium, by using optical spots focused by an objective lens. In the optical recording and/or reproducing apparatuses, the recording capacity depends on the size of an optical spot. The size (S) of an optical spot depends on the wavelength (λ) of light and the numerical aperture (NA) of an objective lens, as expressed by Equation 1:S∝λ/NA  (1)
Accordingly, a short wavelength light source, such as a blue-violet laser, and an objective lens with an NA of 0.6 or greater must be used to reduce the sizes of optical spots focused on an optical disc in order to achieve high-density storage.
If the tilting angle of an optical disc is θ, the refractive index of the optical disc is n, the thickness of the optical disc is d, and the numerical aperture of an objective lens is NA, W31, which is a coma aberration caused by the tilt of the optical disc, can be expressed by a relationship expression such as Equation 2:
                              W          31                =                              -                          d              2                                ⁢                                                                      n                  2                                ⁡                                  (                                                            n                      2                                        -                    1                                    )                                            ⁢              sin              ⁢                                                          ⁢              θ              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                      (                                                      n                    2                                    -                                                            sin                      2                                        ⁢                                                                                  ⁢                    θ                                                  )                                            5                /                2                                              ⁢                      NA            3                                              (        2        )            The refractive index and thickness of the optical disc denote the refractive index and thickness of an optical medium which ranges from a light incidence surface to a recording surface of the optical disc.
Considering Equation 2, a tilt tolerance with respect to the optical disc can be obtained by reducing the thickness of an optical disc as the numerical aperture of the objective lens is increased to obtain a high-density optical disc. CDs have thicknesses of 1.2 mm, and the thicknesses of digital versatile discs (DVDs) have been reduced to 0.6 mm. Next-generation DVDs, called high definition DVDs (HD-DVDs), are high-density optical information storage media having a recording capacity of 20 GB or greater and highly likely to have thicknesses of 0.1 mm. The high-density optical information storage media are presently being developed and standardization to be able to store information about high-definition moving pictures is contemplated. For CDs, the NA of an objective lens is 0.45. As for DVDs, the NA of an objective lens is 0.6. For next-generation DVDs, the NA of an objective lens is highly likely to be greater than 0.6, for example, to be 0.85. When a next-generation DVD is used, a blue-violet light source which radiates blue-violet light with a wavelength of about 405 nm is highly likely to be adopted in consideration of the recording capacity of the next-generation DVD. Accordingly, when an optical information storage medium of a new standard is developed, compatibility with existing optical information storage media will likely be an important consideration.
For example, since a write-once DVD-R and CD-R among existing optical discs has a reflective index which significantly decreases with respect to some wavelengths, light sources which emit light with a 650 nm wavelength and light with a 780 nm wavelength must be used. Thus, to be compatible with today's DVD-R and/or CD-R, an optical pickup for next-generation DVDs needs to adopt two or three light sources which radiate light beams at different wavelengths.
When such a compatible optical pickup using a plurality of light sources which emit light beams at different wavelengths uses only one objective lens, spherical aberrations due to the difference in light wavelength and the difference in the thickness of an optical disc are generated at the same time. Hence, a correction element, such as, for example, a holographic optical element, must be used to correct the spherical aberration.
For example, if the coefficient of the holographic optical element is optimized so that light with a 405 nm wavelength is diffracted as zeroth-order diffracted light and light with a 650 nm wavelength is diffracted as first-order diffracted light to correct for aspherical aberration, the spherical aberrations caused by the difference in light wavelength and the difference in the thickness of an optical disc can be corrected.
However, as shown in FIG. 1, when a holographic optical element is used to correct spherical aberrations, it is difficult to maintain high optical efficiency.
FIG. 1 shows optical efficiency with respect to the depth of a hologram pattern of a holographic optical element of an 8-step blazed type which is manufactured using silica as a base material. As shown in FIG. 1, if the depth of the hologram pattern is set to obtain zeroth order optical efficiency of 70% or greater in the 405 nm wavelength, the first order optical efficiency in the 650 nm wavelength during reproduction of a DVD is only about 10%.
When an objective lens having a high NA, such as, a 0.85 NA is designed and manufactured as a single lens, an advanced technique is required. In addition, it is difficult to manufacture an objective lens which has a high NA to be suitable for both HD-DVDs and DVDs and/or CDs and also has a long working distance like DVD objective lenses.
It is known that when a blue-violet light source and a 0.1 mm-thick optical disc are used, an objective lens requires a working distance of about 0.6 mm. When DVD light with a 650 nm wavelength and CD light with a 780 nm wavelength are focused by an objective lens with a high NA designed to be suitable for a blue-violet light source and a 0.1 mm-thick optical disc, and accordingly optical spots are formed on the recording surfaces of a DVD and a CD, their working distances are 0.32 mm and −0.03 mm, respectively. In other words, the CD and the objective lens collide with each other.
Thus, an optical pickup that includes at least two objective lenses to be compatible with a next-generation DVD and a DVD and/or CD, whose density is lower than the next-generation DVD would be desirable. In this case, a tilt between the objective lenses due to an error upon the assembly of the objective lenses may be generated.
In an optical system having two objective lenses, if a tilt between the objective lenses exists, the skew of one objective lens can be adjusted so that its optical axis is perpendicular to an optical disc, while the other objective lens is tilted with respect to the optical disc.
It is known that if an objective lens is tilted, a wavefront error, particularly, a coma aberration, is generated. Hence, the wavefront error caused by its tilt must be corrected. However, existing objective lenses cannot correct the wavefront error caused by the tilt without the use of additional component elements.
As described above, because existing objective lenses cannot correct the wavefront error caused by tilt without the use of additional component elements, an optical pickup having a single objective lens needs to adjust the skew of the single objective lens during the assembly of the optical pickup if the single objective lens is tilted.
In optical pickups having two or more objective lenses, because optical discs with different recording density have different thicknesses, different working distances are required for each lens. Hence, the difference between required working distances should be considered so as to prevent a collision of an objective lens with a short working distance with an optical disc.
The two or more objective lenses may be mounted on an actuator and driven in a focusing direction and/or tracking direction. Because the number of required objective lenses is two or more, the actuator may be complicated, and a moving part of the actuator may become heavier.